1. Field of the Invention
This invention relates to communications equipment, and more particularly to a method and apparatus for modulating and upconverting to an intermediate frequency electronic signals which represent data symbols.
2. Description of Related Art
Modulation and upconversion of signal are well known in the electronic communication arts. That is, when information is to be transmitted by radio waves, it is common to first modulate a baseband signal with the information to be transmitted and then to upconvert the information from the baseband frequency to an intermediate frequency. Typically the Intermediate Frequency (IF) is then further upconverted to a Radio Frequency (RF) signal for transmission.
The use of an IF frequency in the transmitting of a low cost, small mobile communications terminal is often very desirable by comparison to the alternative of converting directly from the baseband frequency to the transmitter RF frequency. For example, in many mobile or cellular communication devices, communication is full duplex, so that a receiver is operating at the same time as a transmitter. To save parts (and thus power and cost) it is a common practice to use the same synthesizer to provide the Local Oscillator (LO) frequency for both the receiver and the transmitter. But a difficult problem that occurs when direct upconversion is used is that the transmitted signal, which is at a high power level, is at the same frequency as a Voltage Controlled Oscillator (VCO) output in the synthesizer. It is very difficult to provide adequate isolation between the high power transmitter output signal and the VCO to prevent degradation of the synthesizer phase noise performance by the transmitter output. This phase noise degradation results in poor receiver performance.
To prevent this, it is common in the art to use a dual conversion transmitter in which the baseband signal is converted to an IF frequency using a fixed LO, and then a tunable synthesizer is used to upconvert the signal to the selected RF channel. The synthesizer output is offset from the desired transmitter RF frequency by an amount equal to the IF frequency, so the phase noise induced by the transmitter output can easily be removed using a bandpass filter before the synthesizer output is used in the receiver.
A common problem with dual upconversion approaches is that there are many frequency products generated in the second mixer where the IF and synthesizer LO are mixed, and these can fall in the transmitter passband where they can not be easily removed by filtering. It can be shown that in many applications (including cellular voice) when the RF and IF differ by a factor on the order of ten, the unwanted mixer products tend to be spaced sufficiently distant from the desired signal that the most significant products can be readily removed by filtering. Thus, use of an IF frequency of 90 MHz is a very common and desirable approach for cellular applications where the RF frequency is between 800 and 900 MHz.
In accordance with one method for modulating information onto a baseband signal, the phase of an output signal is modulated with a data input signal by a phase modulator. The phase modulator output is equal to s(t)=cos .phi.(t) cos 2.pi.f.sub.c t-sin .phi.(t) sin 2.pi.f.sub.c t=2 cos (2.pi.f.sub.c t+.phi.(t)). The frequency f.sub.c may be either the RF carrier frequency or an intermediate carrier frequency. For the reasons stated above, the frequency f.sub.c is typically an IF frequency and a second conversion is performed to upconvert the output to the RF frequency.
Typically, the output of such a phase modulation scheme is derived from an in-phase component and a quadrature component which are summed at a summing point. For example, FIG. 1 is a simplified block diagram of a common implementation of a GMSK modulator. A digital data input a.sub.k to the modulator is applied via signal line 101 to a GMSK waveform lookup table device 103. The GMSK lookup table device 103 converts the input data into a pair of waveforms, which taken together reflect the state of the last x input data states (where x typically is 3 for binary GMSK with BT=0.5 and modulation index 0.5). A first one of these waveforms is output on signal line 105 and is essentially a digital representation of cos .phi.(t), where .phi.(t) is a phase waveform depending on the input data bit stream. A second one of these waveforms is output on signal line 107 and is essentially a digital representation of sin .phi.(t). These waveforms are then each converted into analog format by a pair of conventional digital to analog converters (DACs) 109. The analog outputs from the two DACs 109 are coupled to two lowpass filters 111. The lowpass filters remove alias energy present at multiples of the sampling frequency that results from the sampling process.
The outputs from each lowpass filter 111 are coupled to a multiplier circuit (such as a mixer) 113 which frequency upconverts the inputs to the IF frequency by multiplying the input by a factor of cos 2.pi.f.sub.c t or sin 2.pi.f.sub.c t. Accordingly, an input signal equal to cos 2.pi.f.sub.c t is coupled to the second input to mixer 113a(i.e., the mixer associated with the in-phase component, i.e., the cos .phi.(t) term). Therefore, the output from the in-phase component mixer 113a is equal to cos .phi.(t) cos 2.pi.f.sub.c t. Likewise, an input equal to sin 2.pi.f.sub.c t is coupled to the second input to the quadrature component mixer 113b and multiplied with a signal equal to sin .phi.(t). Therefore, the output of the quadrature component mixer 113b is equal to sin .phi.(t) sin 2.pi.f.sub.c t. By coupling the in-phase and quadrature components to the positive and negative inputs to a summing circuit 115, the output from the summing circuit 115 is the modulated output s(t)=cos .phi.(t) cos 2.pi.f.sub.c t-sin .phi.(t) sin 2.pi.f.sub.c t. This output is typically frequency upconverted one more time to the required RF frequency. The RF upconverter is not shown.
One problem associated with such modulation schemes is that a relatively large number of analog components must be used. That is, there is a need for two DAC circuits, two analog lowpass filters, two analog multipliers, and an analog adder. It should be clear that a method and apparatus for modulating and upconverting the signal output from a modulator to provide an output equal to cos .phi.(t) cos 2.pi.f.sub.c t-sin .phi.(t) sin 2.pi.f.sub.c t without the need for all of the components which are presently required would be desirable. This is because these components require additional space and power, and increase the cost of the system. In small portable communication devices, size, cost and power consumption are of great importance.